Circuit board test fixtures are devices which are used in testing circuits and individual components of a printed circuit board, often referred to as a "unit under test" or "UUT". In use, the fixture generally holds and positions the UUT, and establishes electrical interconnections between test equipment and test points or nodes on the UUT. The test equipment typically includes signal circuitry for generating a selected excitation signal applied to the UUT test points through the fixture interconnections, and for detecting or monitoring a response signal therefrom as part of a manufacturer's quality assurance testing program or as a diagnostic procedure during circuit board repair and servicing.
One type of test fixture is the so-called "bed of nails" device, which, as the name suggests, has an array of spring-loaded contact probes which support and make electrical contact with the UUT. The probes are mounted at fixed locations in an interface plate in registration with all desired test points on the circuit board. The probes are electrically wired at their other ends to the test signal circuitry. During testing, the UUT is disposed in the test fixture, which is typically within or equipped with a press for the application of pressure to make the requisite probe-to-UUT contacts.
Although known versions of such test fixtures are generally suitable for their intended purpose, certain inherent limitations have been identified. One such limitation stems from the fact that each different UUT will generally have test points arranged in a grid pattern unique to that UUT. For testing, the fixture's probes are arranged in the identical grid pattern. To appreciate how this can be problematical, consider a typical test fixture having a field of a thousand probes mounted in the interface plate on 100 to 200 mil centers, and in accurate registration with the UUT test points. The interface plate has to be designed and manufactured, and all probes individually wired, specifically for the particular UUT. If any modification is made to the printed circuit board components or circuitry that necessitates a change in the location of its test points, the test fixture must be redesigned and often rebuilt to account for the change in the grid pattern. Then the probes must be again wired to the test circuitry. This can be a costly matter for typical test fixtures used today.
Needless to say, a different test fixture is typically designed, built and stored (when not in use) for each circuit board layout to be tested. The attendant costs can be quite high. Desirable is an approach to test fixture design which affords more flexible, versatile and long-lived test fixtures. Such improved fixtures should lend themselves to more inexpensive and efficient design, fabrication and modification to accommodate any of a wide variety of probe patterns.
More-elaborate fixturing systems have been proposed to provide this flexibility, for instance, using wire wrap technology. In Matrone et al., U.S. Pat. No. 4,230,985, a fixturing system is described in which the UUT is mounted on a "bed of nails" type unit, called an "access unit". The access unit has a universal matrix platen provided with a plurality of spring-loaded, conductive test probes extending from the side of the top plate facing the UUT for pressure contact with the UUT test points. Each test probe also extends beyond the other side of the platen so as to provide a test post. These test posts are wire-wrapped to a fixed field of contact posts mounted in a contact panel, which is spaced inboard from the platen and disposed on the bottom of the access unit. The contact posts are shown as being located beneath a side edge of the platen. In other words, the contact posts are laterally offset from the field of test probes and posts. The contact panel makes individual electrical connections between the test circuitry system and the test probes contacting the UUT.
This arrangement suffers disadvantages for certain applications. The Matrone system unfortunately requires relatively long wires in the test path connecting the probes to the laterally-offset contact posts. The resultant duration of signal reflections caused by impedance mismatches between the UUT and the test circuitry can adversely affect test results. Also, as indicated above, this arrangement requires wire wrapping both on the bottom side of the platen and on the top side of the contact panel. To require a wire wrapping tool to access those plates from opposite sides needlessly complicates the wiring process.
U.S. Pat. No. 4,099,120 to Aksu also describes a fixturing system with a laterally displaced contact field. In Aksu, a plate head provides a "probe contact bearing area," on which the test unit is mounted, and a laterally displaced "interfacial contact bearing area," which includes a fixed field of contact posts which engage a corresponding field of spring-loaded probes connected to the test apparatus. The test probes in the first area are individually wired to an associated contact post in the second area. Although this system advantageously provides for all wiring to be done on a single side, the wiring to the laterally displaced second area is quite lengthy, and therefore significantly adds to the length of the test signal paths.
A further known design for fixture devices employing wire wrapping techniques also suffers certain disadvantages. In this design, a bed-of-nails type interface having spring-loaded probes mounted in a fixed two-dimensional array is positioned over circuitry of a test system. A contact board is positioned above the interface board, and includes an array of contact pins for making electrical contact (i.e., pressure contact) with the probes of the interface board. Each contact pin extends through the contact board, terminating in a wire-wrappable contact post. These contact posts are wired to the undersides of intermediate posts extending through an intermediate plate. The intermediate post tops are, in turn, wired to the test probes of a typical bed-of-nails probe plate positioned under the UUT. This fixture device's design requires wiring on a single side of several boards. The wires are relatively long, having to run to and from the interposed array of contact posts. As such, the cumulative effect of this wiring arrangement can detrimentally reduce the speed and efficiency of the circuit board tests.